Method of making formed products of cellulose crystallite aggregates



y 1966 o. A. BATTISTA ETAL 3,259,676

METHOD OF MAKING FORMED PRODUCTS OF CELLULQSE CRYSTALLITE AGGREGATES Filed Jan. 8, 1963 WET CELLU LOSE CRYSTALLITE AGGREGATES DISINTEGRATE DRY PRESS DRY PRESS PRESS AND DRY DlSINTEGRATED AND ASSOCIATED CELLULOSE CRYSTALLITE AGGREGATES United States Patent 3 259 676 METHOD or MAKIN G i oRMEn PRonUcrs or CELLULOSE CRYSTALLITE AGGREGATES Orlando A. Battista, Drexel Hill, and Patricia F. Smith, Parlrside, Pa., assignors, by mesne assignments, to FMC Corporation, San Jose, Calif., a corporation of Delaware Filed Jan. 8, 1963, Ser. No. 250,026 7 Claims. (Cl. 264123) This application is a continuation-in-part of copending application Serial No. 27,268, filed May 6, 1960 which is a continuation-in-part of application Serial No. 823,081, filed June 26, 1959 (now abandoned).

This invention relates to making formed products of cellulose crystallite aggregates, and more particularly of mechanically disintegrated cellulose crystallite aggregates having a level-off D.P. (Degree of Polymerization).

As described below, formed products are obtainable having a thermal resistance and physical strength of magnitudes not heretofore observed in cellulosic materials. The products are of further interest in respect of their high density, good electrical properties, flexibility of formation, and fabrication ease.

Essentially, and in brief, the invention comprises making formed plastic structures comprising mechanically disintegrated cellulose crystallite aggregates disposed sulficiently close together that the aggregates are interfacially bonded by means of intermolecular bonding forces.

The drawing is a block diagram illustrating the methods of converting or shaping the cellulose crystallite aggregates into heat and impact-resistant self-supporting unitary three dimensional structures.

Cellulose crystallite aggregates are small disintegrated aggregates of level-off D.P. cellulose. These small disintegrated aggregates, their properties and a method of disintegrating the level-off D.P. cellulose are described in United States Patent 2,978,446, issued April 4, 1961. They are acid-insoluble products produced by the controlled acid hydrolysis of cellulose and the level-oil D.P. value reflects a destruction of the original fibrous structure of the cellulosic source material. The term level-oil D.P. has reference to the average level-off degree of polymerization of the cellulose product measured in accordance with the paper by O. A. Battista entitled Hydrolysis and Crystallization of Cellulose, vol. 42, Industrial and Engineering Chemistry, pages 502*7 (1950).

As described in the foregoing patent, a cellulose material is subjected to a controlled acid hydrolysis to dissolve amorphous forms of cellulose leaving as a residue cellulose crystallite aggregates. The cellulose is subjected to the hydrolysis treatment for a time and at a temperature sufficient to disrupt'the continuity of the fine structure. One such method comprises treating the cellulose with a 2.5 normal hydrochloric acid solution for minutes at the boiling temperature of the solution. Another suitable method involves treating the cellulose with a 0.14 normal (0.5%) hydrochloric acid solution at 121 C. under pressure for 1 hour. In accordance with the method as described in the patent, the insoluble residue is preferably washed free of acid.

The cellulose crystallite aggregates which are recovered from the hydrolysis treatment have a particle size in the range of from 1 or 2 microns to about 300 microns as determined visibly by microscopic examination and may be designated as as-formed cellulose crystallite aggregates. As described in the patent, by subjecting a mass of crystallite aggregates to a mechanical disintegration so as to produce a mass of disintegrated aggregates wherein at least about 1% of the cellulose crystallite aggregates, by Weight, have a particle size less than about 1 micron, the disintegrated mass is capable of being colloidally suspended in aqueous media; that is, the mass is characterized by forming a stable suspension in aqueous media.

The source material for the cellulose crystallite aggregates may be any of the natural cellulose materials, such as natural fibers, for example, ramie, cotton, purified cotton, purified wood pulps such as bleached sulfite pulp, bleached sulfate pulp, etc., or regenerated forms of cellulose, -for example, rayon, cellophane, etc., and in all instances has an actual D.P. greater than its average leveloii D.P. The aggregates may have an average level-oif D.P. value in the range of from about 15 to about 375 and the specific value will be dependent upon the source material. Aggregates having an average level-01f D.P. in the range of 15 to 6 0, for example, are produced from regenerated forms of cellulose. Aggregates having an average level-off D.P. in the range of 60 to 125 may be obtained from alkali swollen natural forms of cellulose such as cotton linters and purified wood pulps. Sulfite pulp as a source material will produce cellulose crystallite aggregates having an average level-oif D.P. in the range of 200 to 390. Although the cellulose chains in the crystallites are uniform in length by comparison with the source material, some variation occurs, and, for this reason, the D.P. is referred to as an average level-off D.P. value.

In accordance with recognized principles of polymer chemistry, in speaking of degree of polymerization (D.P.) reference is made to the pure polymer. It is to be understood that the stated level-off D.P. values of the cellulose has reference to substantially pure cellulose such as cellulose crystallites derived from highly purified cellulose sources including regenerated forms of cellulose. Where a crude or raw wood pulp is subjected to a hydrolysis treatment for the production of the level-oil D.P. cellulose crystallites, appreciable amounts of lignin and other non-cellulose materials will be present in the residue and measurements of average level-oil DR of such residue may exhibit apparent values as high as 500. Such high values reflect the presence of lignin and other non-cellulose material and are not true values of the level-01f D.P. of the cellulose per so. However, there is present in the mass some cellulose which has been reduced to the average level-ofl D.P.

Derivatives of cellulose crystallite aggregates which are essentially topochemical derivatives possess physical characteristics and properties similar to those of cellulose crystallite aggregates and may be used .in the practice of the invention. For example, the derivatives have about the same D.P. and a size in the same particle size range as the crystallite aggregates from which they have been formed, and the disintegrated derivatives h3JV dispersibility and gel-forming characteristics similar to those of the disintegrated cellulose crystallite aggregates. Chemically, the degree of substitution DS) is at least 0.01. The D.S. has reference to the total degree of substitution which may include both hydrophilic and hydrophobic substit-uents. The derivatives may be formed from disintegrated cellulose crystallite aggregates and will have a particle size ranging from below 1 micron to about 250 microns depending directly upon the particle size of the aggregates. Alternatively, the derivatives may be formed from non-disintegrated aggregates and subsequently disintegrated. Where the D8. is sufiiciently low so that the derivatives are water-insoluble and/or organic solventinsoluble, thedisintegrated aggregates derivatives have a unique and distinguishing characteristic, like that of the disintegrated crystallite aggregates, in their ability to form stable dispersions in non-solvent liquid media, which media is capable of partially swelling the disintegrated aggregates, providing that the derivatives constitute at least about 0.5% by weight of the dispersion and at least 1% of the Weight of the dispersed aggregates derivatives have a particle size not exceeding about 1 micron.

Patented July 5, 1966 The derivatives containing hydrophilic substituents are water-sensitive and the ease with which aqueous dispersions or suspensions may be formed is dependent directly on the water-sensitivity of the derivative. For example, a stable aqueous dispersion of a water-insoluble hydroxyethyl derivative may be formed more easily and readily than a corresponding stable dispersion or suspension of the cellulose crystallite aggregates because the hydroxyethyl cellulose is more sensitive to water than the cellulose crystallites themselves. The appareance and nature of the dispersion or suspension will also vary with the Water-sensitivity of the derivative. Where the D.S. of a derivative having a hydrophilic substituent is not more than about 0.1 to about 0.2, the dispersion of the disintegrated derivatives will have about the same appearance as -a corresponding dispersion of disintegrated cellulose crystallite aggregates. For example, dispersions or suspensions of disintegrated sodium carboxymethyl or hydroxypropyl derivatives of the aggregates having a D.S. of about 0.1 have the same white glistening appearance as a corresponding dispersion of the disintegrated aggregates. As the D.S. of the disintegrated derivative is increased, as for example to a D.S. of about 0.3, the dispersion has a more translucent appearance and where the D.S. is about 0.4, the dispersion has an appearance approaching that of white petrolatum.

Where the aggregates derivatives contain hydrophobic substituents, the disintegrated derivatives may be dispersed or suspended in various organic solvent systems in which they are insoluble but in which they are partially swollen. In forming stable dispersions of this group of derivatives, the ease of formation of a stable dispersion or suspension will vary with the sensitivity of the derivative to the organic solvent and the appearance of the dispersions will vary with the degree of substitution in a manner similar to that of the aqueous dispersions of the water-sensitive derivatives. For example, a stable dispersion (1%) of the disintegrated acetate derivative ('D.S.2.0) may be formed in acetone. Similarly, a stable dispersion of 1% (by weight) ethyl cellulose (D.S.l.5) may be formed in methyl ethyl ketone.

A wide range of derivatives of the cellulose crystallite aggregates are satisfactory for the purposes of this invention and these derivatives are disclosed in eopending application Serial No. 2,133, Serial No. 2,134 and Serial No. 2,135, all \filed January 13, 1960. These derivatives include, for example, oxidation derivatives containing '1 or more carbonyl linkages including aldehyde, carboxyl and mixed aldehyde-carboxyl derivatives; ether derivatives characterized by having one or more OR groups where R may be an aliphatic or substituted aliphatic radical, either a straight or branched chain containing from 1 to 12 or more carbon atoms, an aryl or .aralyl or substituted aralkyl radical or a carboxyalkyl, hydroxyalkyl, cyanoalkyl, 'alkoxyalkyl, aralkyloxyalkyl, or a dialkylaminoalkyl radical; and ester derivatives, for example, esters of inorganic acids such as the nitrates, nitrites, thiocyanates and phosphates and esters of organic acids such as the form-ates, acetates, propionates, buty-rates, mixed acetatepropionates, tmixed acetate butyrates, other aliphatic acid derivatives containing up to :18 or more carbon atoms and aryl or aralkyl esters such as, for example, benzoates, phenyl acetate esters, phthalate esters, naphthenates and the like.

For the purposes of the present invention, the cellulose crystallite aggregates and the Water-insoluble and/or organic-solvent insoluble aggregates derivatives having physical characteristics and properties similar to those of the cellulose crystalli-te aggregates are equivalents. In the succeeding discussion, and the claims the terms cellulose crystallite aggregates and aggregates is used to designate both the cellulose crystallite aggregates and the aggregates derivatives.

As obtained from the acid hydrolysis and water washing steps, the aggregates in the over-all average level-off D11. range of 15 to 375 are in a loosely aggregated state and, particularly in the larger sizes, say from 40 to 250 or 300 microns, are characterized by the presence of many cracks in their surfaces, including similar surface irregularities or phenomena like pores, depressions, voids, fissures and notches. Because of such irregularities, the apparent or bulk density of the aggregates is much less than their absolute density. Furthermore, the cracks and other irregularities persist despite the application of high compressive forces on the aggregates. Thus, when they are compressed at 5,000 p.s.i., they exhibit an apparent specific gravity of 1.26; at 10,000 p.s.i. the apparent specific gravity rises to 1.32; at 15,000 p.s.i. it is 1.34; and at 25,000 and 37,000 p.s.i. it is 1.38 and 1.38, respectively. On the other hand, the absolute specific gravity of a unit crystal or crystallite is 1.55 to 1.57, from which it is apparent that the aggregates occlude considerable quantities of air in the surface cracks, voids, fissures, etc. The apparent specific gravity of the dried disintegrated aggregates at the compressive forces noted, are somewhat higher than the foregoing values.

The aggregates resulting from the hydrolysis and Washing steps are further characterized by having a particle size in the range of 1 or 2 to 250 to 300 microns, as determined visibly by microscopic examination.

In most cases it is preferred to subject the foregoing product to mechanical disintegration, as described below, there being produced a material Which, generally speaking, has a size in the range of at least 1% by weight of less than 1 micron to about 250 or 300 microns. The disintegrated aggregates are then subjected to the product forming step.

Mechanical disintegration of the aggregates may be carried out in several Ways, as by subjecting them to H attrition in a mill, or to a high speed cutting action, or

they may be forced through a multiplicity of fine holes so that they are subjected to an attriting or shearing action by entrance and passage through the openings as well as to the attriting action by virtue of being rubbed against each other under the influence of applied force. The disintegration of the aggregates may be carried out in the presence of an aqueous medium. Whatever method is used, the disintegration is carried out to such an extent that the resulting disintegrated aggregates are characterized by forming a stable homogeneous suspension in the aqueous medium in which they are being attrited, or in which they may be subsequently dispersed. By a stable suspension is meant one from which the aggregates will not settle out but Will remain suspended indefinitely. The disintegrated aggregates are further characterized by the fact that such suspension forms an extremely adherent film when deposited on a glass panel or sheet or other suitable surface. At lower concentrations of aggregates, the suspension is a dispersion, while at higher concentrations it is a gel.

As described hereinafter, the product forming step is preferably carried out by using pressure, and it may be noted that disintegration or attrition of the aggregates also takes placing during the step of compressing them, particularly of the larger, randomly disposed aggregates, at least 1% of which by weight are reduced to a size of the order of one micron and smaller. In fact, the greater the pressure, the more particles will be obtained having a size of less than one micron. At the same time, compression has the effect of orienting, or lining up, the small particles so that the faces of these latter aggregates are brought so closely together that the aggregates are held by intermolecular bonding forces, and a larger structure is formed. The density of such larger structure, or compressed shape, tends to approach the density of the crystallite aggregates. To facilitate the foregoing actions, and to increase the particle to particle contact, the compression of the aggregates may be carried out in a vacuum of any suitable magnitude.

The process of producing the formed products general- 1y comprises attriting the aggregates either prior to, and/or during, the product forming operation. When wet a drying step usually follows the forming, although it may be carried out coincidently with or, less preferably, prior to the forming. The pressure employed is such or the other attrition is of such character and duration that at least 1% by weight of the cellulose crystallite aggregates or derivatives are reduced to a particle size less than one micron.

Generally, the forming step is accomplished in the cold, i.e., at room temperatures, by applying pressure, the amount of which is at least 5,000 p.s.i., although preferably it is 15,000 to 30,000 p.s.i., and more preferably 30,000 to 100,000 p.s.i. As the forming pressure increases, the density and hardness of the resulting products increase, and it progressively becomes more insensitive to moisture.

The forming pressure may extend to pressures on the order of 1 or 2 to 5 to 7 million p.s.i., in which case explosion compression or explosion compacting techniques are employed. At these last-mentioned pressures, the specific gravity of the product approaches the limiting value of about 1.57, which is the absolute specific gravity of a unit crystal or crystallite. The hardness of the product is very great, and in other respects the product is quite unusual. Owing to its great heat resistance, hardness, and impact strength, its use as a mold is contempleted, particularly where great resistance to heat and pressure are desired.

The drying step may be carried out at temperatures ranging from room to about 100 C., preferably at 20 to 30 C. Drying conditions which tend to produce steam should be avoided. Usually, the less water left in the pressed product, the greater is its resistance to moisture.

A number of more or less specific procedures are available for making the product. According to a preferred method, wet or never dried crystallite aggregates are subjected to mechanical disintegration or attrition, then to a pressing step, and the pressed product is then dried. Preferably the aggregates are pressed under conditions that will permit at least a portion of the water to escape therefrom, as by at least partially enclosing them by means of a water permeable member or device. The resulting product is usually at least translucent and it approaches transparency as the particle size of the aggregates becomes smaller. The product is characterized by having a very uniform structure, and may be made extremely tough and hard. It has good resistance to moisture. Usually the surfaces of the product are glossy, and they are capable of receiving an ever glossier finish as by subjecting them to a suitable bufling operation.

In the foregoing procedure, the aggregates resulting from the attrition step may have an over-all particle size in the range of less than 1 to 250 or 300 microns, and this material may be subjected to the pressing step. It is possible, and in many cases is a desirable procedure, to fractionate the attrited aggregates before compressing, and thus press, say, a fraction having particles of up to 1 or 2 microns, or up to 5 or 10 microns, or of any other desirable size and size distribution. Furthermore, the aggregates-water mixture subjected to attrition, and prior to pressing, may have a consistency of 3 to 90% solids, and preferably 35 to 80 or 90% Another preferred procedure comprises subjecting the Wet or never-dried aggregates, having any desirable particle size and size distribution, to compression, preferably under conditions permitting the escape of water, and then drying the pressed product, there being obtained a hornified material having a very smooth, glossy surface.

Still another preferred method comprises attriting the wet as-formed aggregates, say at a consistency of 5 to 35% solids to form a wet gel, and thereafter drying the gel at atmospheric pressures to produce a tough, hornified product. Similarly, an aggregates-water mixture of 35 to 80 or 90% consistency may be attrited, and the resulting mixture dried to yield a particulate form, more or less, of highly hornified pieces or pellets of variable shape; they are of value as non-conducting material in circuit breakers.

In still another variation, the never dried aggregates may be attrited and then subjected to a coincident pressing and drying step. In this case the product is not only hornified but has an ivory-like or pearly appearance.

In another procedure the dried aggregates, disintegrated or not, may be subjected to pressure, there being produced a product which, while not glossy or pearly, is nevertheless useful.

Generally, the wet aggregates may be compressed at lower pressures than the dried to achieve a comparable bonding effect in the product. Also, pressing may be done at higher temperatures if desired, say room to or C., or more, to favor hydrogen bonding, but is conveniently performed at room temperatures. The aggregates do not flow under the influence of pressure.

While the foregoing procedures provide products having a gradation of properties, all products have several common features. Thus, all are unitary 3-dimensional plastic materials. All comprise attrited and dired aggregates. In all of them, the aggregates particles, because of the purity of the crystallites and the forcing into close association by means of pressure or surface tension, of the free hydroxyl groups on their surfaces, are considered to be interfacially bonded. More particularly, and without being bound by theory, it is considered that the crystallite planes are sufficiently clean to permit very intimate lateral packing of the crystallites, so that intermolecular bonding occurs; for example, hydrogen bonding forces between adjacent lateral OH groups on the crystallite surfaces become engaged, and van der Waals forces are believed to be involved. It is thought that these intermolecular bonding forces are predominant at pressures above 25,000 p.s.i. and densities above 1.4; in any case they help account for the great cohesiveness of the product; moreover, such forces cooperate with mechanical interlocking forces, which are prevalent at lower pressures, to give a product which is quite dense, extremely strong and hard, and unusually resistant to high temperatures. For example, pressed discs in their as-formed state, made as described, can be pounded with a hammer without breaking, and they will not receive nails; the nail bends under the hammer rather than penetrate the disc. Furthermore, a half-inch disc can withstand contact by an oxyacetylene flame (3100 to 3300 C.) for periods of 12 to 20 seconds without being pierced. The flame resistance increases as the disc thickness increases. Furthermore, there is very little conductivity of heat to the underside of the disc. The flame gradually destroys the material of the disc but only at the point of contact. The material does not melt. In the arc plasma, (10,000 K.), a half-inch thick disc pressed at 10,000 p.s.i. exhibits an erosion rate (2.4) which is less than that of polytetrafluoroethylene or Teflon (6.4), or of vulcanized fiber (2.7), and moreover, carbon is not deposited on it.

It may be observed that the specific gravity of products pressed at 10,000 p.s.i. is about 1.30 to 1.39 and at 25,000 p.s.i. it may be 1.38 to 1.43. Other preferred ranges are 1.0 or 1.2 to the absolute specific gravity of a cellulose crystallite, namely 1.54-1.57. The Izod impact strength of a bar pressed at 25,000 p.s.i. is in the range of 0.75 to 1.0 ft. lb./in. of notch, and, for more highly pressed products, may range to 5, 10, or more ft. lbs/in. The heat conductivity of a product pressed at 10,000 p.s.i. is 1.75 B.t.u./in./sq. ft./ F./hr., while the specific heat is 0.4 B.t.u./lb./ F. It may be noted that these characteristics are superior to those of vulcanized fiber.

As an example of the hardness, a material pressed at 37,000 p.s.i. has a hardness which is comparable to that of conventional vulcanized fiber and superior to that of a conventional commercial molded product made from about 65% urea-formaldehyde resin and about 35% alpha-cellulose as filler. The less hard materials are readily machinable on conventional equipment.

The products have useful electrical properties; of example, for a material pressed at 10,000 p.s.i. the dielectric constant is 6.1 at 57% RH, 75 F., 1,000 c.p.s., and 56.4 at 95% RH, 75 F., 1,000 c.p.s.; and the dissipation factor is 0.031 at 57% RH, 75 F., 1,000 c.p.s, and 0.402 at 95% RH, 75 F., 1,000 c.p.s.

Some of the products particularly those formed by pressing wet aggregates and then drying, acquire a glossy ivory-like appearance and are bulfable to an even higher gloss. These particular products tend to be more water resistant; when immersed in water they retain their form and shape for a considerable period, in time becoming somewhat rubbery to the touch before eventually disintegrating. They are resistant to humid atmospheres.

The compressed aggregate products are of value in applications requiring great thermal resistance and outstanding strength, as for example in the wall portions, including the nose cones, of guided missiles, preferably as an intermediate layer. A further advantage for these and other installations is that the products may be made ,of any suitable desired thickness. Other useful applications include thermal insulating materials in building, automotive, and equipment construction, safes, and the like.

In respect of moisture resistance it is interesting to note that the aggregates, in powder form, possess the lowest moisture regain of any known form of cellulose, namely, 5.7% moisture regain at 58% relative humidity and 72 F. as compared with up to 13% moisture regain for other forms of cellulosic materials. At 40 and 80% RH, the aggregates have a moisture regain of 4.2 and 8.4%, respectively. When pressed into a disc at 10,000 p.s.i., the moisture regain of the aggregates is 3.8% at 58% RH and 72 F. The moisture resistance increases with the forming pressure.

To maintain the pressed products in a moisture resistant state, and to preserve their properties of heat resistance, physical strength, dielectric strength, etc., they should be protected against prolonged exposure to humid atmospheres, moisture, dampness, etc. While they have value in non-protected installations, such value and use is not permanent. However, in protected installations their value and use are long lasting. For example, in thermal applications the products may be externally covered by a suitable layer or coating which is permanently Water resistant, such as an aluminum, steel, or other metal sheet or sheath, or a resin coating, for which there may be used any of the resins described below.

In some cases, the pressed product may comprise an intermediate layer or wall of a given structure, such as a safe; that is, in its normal use it may be disposed between a pair of layers or walls that are moisture resistant, and thus it will not require special handling.

Increased moisture resistance of the product during exposure to the atmosphere may also be obtained, without any appreciable sacrifice of the other noted properties, by mixing the cellulose crystallite aggregates, prior to forming of the product, with a minor amount of suitable moisture-resistant resin. Greater moldability also results. Incorporation of the resin may be made before or after attriting the aggregates, but should of course precede the pressing or forming step, which in this event is done at elevated temperatures as conventionally employed in resin molding procedures. The amount of resin should be sufiicient to provide moisture resistance and suitably may range from 0.01 to 0.5 or 1 or 2%, although in some cases it can be higher, say to 5, 10, 15, 20 or 25%. By employing smaller amounts of resin, the heat resistance, hardness and impact strength of the pressed product are retained, particularly when such heat-resistant resins like polytetrafiuoroethylene, polyformaldehyde, polyamides, etc., are used. Addition of the resin to the crystallite aggregates will usually require the application of heat coincidently with pressure in order to form the products. The resin may be a thermosetting or heat-hardenable resin, usually in the form of a precondensate or partial polymerizate such as phenol-formaldehyde resins. Other resins are alkyd resins; also protein plastics such as those made from casein and zein; epoxy resins; vulcanized polyisobutylene, etc. Thermoplastic resins include polyamides like nylon, polyurethanes, polyformaldehyde, polyolefins such as polyethylene, polypropylene, unvulcanized polyisobutylene, furans, fluoroethylene polymers, silicones, etc. The preferred resins are those which soften at 200 C. or higher.

Mixing of the aggregates with the resin may be carried out in any suitable way that will secure effective distribution and intermingling of the materials. In the case of thermosetting resins, liquid precondensates or resin syrups are usually employed and are made in the customary way. For example, formaldehyde and phenol are reacted in the presence of a suitable polymerization catalyst to form a resin syrup, and this is mixed with the cellulose crystallite aggregates. The resulting mixture, in the form of a hard lumpy material is disintegrated in a mill or other device to reduce the size, then dried, and it may be further reduced in size as by grinding in a ball mill or the like. Drying of the aggregate-resin mixtures may be done at temperatures up to, say, 150 C. for times ranging from a few seconds to a few minutes. The times are shorter at the higher temperatures. When the resulting composition is pressed and heated, the resin is brought to a fully condensed or cured state.

If desired, resins like phenol-formaldehyde may be condensed to solid form, reduced to a friable powder, mixed with the aggregates, and then blended on rolls, after which the mixture may be reduced to suitable size.

Thermoplastic resins may first be ground to powder, mixed with the aggregates, then blended on warm rolls to form a sheet, and the latter is then cooled and reduced to powder or granules.

It is practical to dissolve the resin in a suitable solvent and then to mix the resulting solution with the aggregates; or the dissolved resin may be precipitated on the aggregates. Another mixing procedure is to combine the aggregates, either in the form of a dispersion or a wet gel or attrited mixture, or a fraction thereof, obtained as described, with a resin syrup or resin solution, then mix the materials, and thereafter dry the mixture. Still another, and preferred, mixing procedure is to attrite or disintegrate the aggregates directly in the resin syrup or resin solution, or other form of liquefied resin, rather than in the aqueous medium as noted above, and subsequently to dry the attrited mixture. Very intimate mixing is provided by this last procedure.

The invention may be illustrated by the following examples.

Example I Viscose fiber was subjected to hydrolysis in a 2.5 normal HCl solution for 15 minutes at C., filtered, and then washed with water until it was acid-free. The resulting crystallite aggregates had an average level-off DR of 40. The moisture content was about 5%. A mixture comprising 50% by weight of the aggregates in water was prepared having a particle size ranging from 1 to about to 200 microns. Then a bronze, sintered metal, cylindrical mold equipped with a tight-fitting piston was mounted on the lower plate of a Carver press and 2 ml. of the aggregates-Water mixture were placed in the hollow core or cavity of the cylindrical mold beneath the piston and pressure was applied to the piston by forcing the upper plate of the press against it. A pressure of 10,000 psi. was applied, and it was noted that the water present in the mixture Was forced through the porous walls of the mold, appearing on the outer surface. The pressure was relieved after a value of 10,000 p.s.i. was reached, and the material in the mold cavity was found to be in the form of a compressed paste or moist cake. The particle size of the paste was found by microscopic examination to be up to 3 microns. A portion of the compressed paste was dried in air, there being formed a translucent horn-like mass that was extremely hard and had the appearance of artificial ivory.

Example 2 A sample of regenerated cellulose obtained by the deacetylation of cellulose acetate (Fortisan) was subjected to hydrolysis, using the conditions described in the preceding example, there being produced crystallite aggregates having an average level-off DP. of 60. They were dried to a moisture content of about This material was thoroughly wetted with water so that the moisture content was about 50%, and the wet mixture was placed in the cavity of a cylindrical mold equipped with a tight-fitting piston and subjected to a pressure of 10,000 psi. in the Carver press, there being produced a wet block or cake that could readily be broken by hand. Examination of the pressed cake under the microscope showed that substantially all of the particles had been reduced to a size of less than microns. On removing the cake and drying it at room temperature, there was obtained a horn-like product having smooth ivory-like surfaces. It was noticed that a continuous translucent film was left adhering to the piston, and this film was also collected and dried, yielding a translucent, extremely hard, horn-like product having ivorylike surfaces. Both products were found to have good moisture resistance. When immersed in Water, they remained intact for several hours, i.e., they did not disintegrate; rather, they tended to become plasticized in that they became somewhat rubbery to the touch. Both products were readily machinable, and their surfaces capable of taking on a high gloss.

Example 3 Cellulose crystallite aggregates were prepared by bydrolyzing Ketchikan sulfite wood pulp with 0.5% by Weight HCl for 60 minutes at 250 F. The aggregates were washed once with deionized water, once with a 1% ammonium hydroxide solution, again with deionized water, after which they were dried in an oven at 65 C. for 24 hours under a vacuum of 29 inches of water. They had an average level-off DR of 220, a moisture content of about 5% by weight, and a particle size ranging from about 1 to 250 or 300 microns. A number of pressed discs were made, measuring 2% inches in diameter and $5 inch thick, by pressing the dried powdered aggregates at room temperature and at 10,000 psi. in a simple mold positioned in a Carver press. Pressing increased the bulk density of the aggregates from approximately 0.3 for the powder to about 1.38 for the pressed discs. The discs were white, and very hard, the hardness being comparable to a molded article made from a molding composition comprising about 65% ureaformaldehyde resin and 35% alpha-cellulose filler. It was not possible to hammer a 3-inch building nail through a disc; the nail bent under the impact of the hammer rather than penetrate the disc. The discs had a heat conductivity value of 1.75 B.t.u./in./sq. ft./ F./hr., a specific heat of 0.4 B.t.u./lb./ F., and a density of 80 to 90 lbs/cu. ft. When subjected to the full blast of an oxyacetylene torch -(31003300+ C.) for a period of seconds, a disc of crystallite aggregates withstood the flame substantially better than discs and other shapes made of nylon, transite, Marinite, steel, hard wood, soft wood, or porcelain. When removed from the flame, the disc of aggregates had, at the point exposed to the flame, a central cavity in one side about /2 inch in diameter and extending approximately half way through the thickness of the disc. The area surrounding the cavity was flat and covered by a thin carbonized layer which, upon cutting the disc diametrically in half, was shown to be simply a surface eflect. When viewed in cross-section, and cornpared with a control disc, each cut surface of the flametreated disc halves appeared to be normal, apart from the cavity produced by the flame, and had an intensely white, opaque appearance. It was evident from the cut surfaces in the vicinity of the cavity that carbonization of the material had proceeded only layerwise, disintegration of the product. It was noted that at the end of the 15-second exposure to the flame, the disc could be picked up immediately by grasping its edges by the fingers, and that on the side remote from the flame-exposed side, the disc was only warm, not hot, to the touch. This behavior was attributed to its very low heat conductivity. The nylon disc, measuring 2 /2" by become white hot after 5 seconds, two large central holes being formed which pierced the disc, while the side exposed to the flame was concaved, indicating a flow of the material. A marinite square, comprising pressed asbestos, 3" by 3" by had a hole burned through after 5 seconds with some indication of flow. A transite disc, 2%" by 2" was burned through after 5 seconds. It was also made of asbestos. Both the mariniate and the transite could be taken up in the hand, after removal of the flame, by grapsing the pieces by the edges. A rectangular steel plate, 3 /2" by 2" by showed a central cavity about deep after only 3 seconds exposure to the flame; flow occurred, and the piece was white hot. Teflon, comprising a polytetrafluoroethylene disc 2 /2" by was not pierced by the flame after 15 seconds, although a cavity about deep was formed. There was evidence of flow. A piece of vulcanized fiber was pierced after 5 to 10 seconds. Porcelain in the form of a square, 4" by was not pierced after 10 seconds; however a cavity was formed about A" deep and there was evidence of flow. A piece of hard wood in the form of a disc, 2%." by was pierced after 5 seconds and the exposed side of the piece burst into flames. The opposite side showed charring. A piece of soft wood was even more extensively damaged after 5 seconds.

Example 4 Cellulose crystallite aggregates were prepared as in the preceding example and several discs were cold pressed, using a pressure of 35,000 to 40,000 psi. The discs were about 1 /2 inches in diameter and /2 inch thick. They had a bulk density of 1.32 to 1.38, were very hard, white and opaque. The mold was provided with slight, vertically extending ridges, and the disc was able to reflect these ridges accurately, there being formed in its edges corresponding grooves.

Example 5 Cellulose crystallite aggregates were prepared by bydrolyzing Ketchikan sulfite wood pulp with a 0.5% by weight aqueous solution of HCl for 1 hour at 250 F., there being obtained a never dried material having an average level-01f DR of 220 and a moisture content of 63.9%. This material was then made up into an aqueous mixture comprising 32% by weight of the aggregates and the balance water, and the mixture subjected to attrition in a stainless steel sigma blade mixing device. This comprised a rectangularly-shaped compartment having an open top and provided at the bottom with two side-byside concaved portions separated by an upstanding ridge;

A sigma blade was adapted to rotate in each concaved portion, one end of each blade being mounted to a side of the compartment and the other end being unmounted but disposed closely to the opposite side of the compartment. The clearances of the blades with the end surfaces of the compartment and with the surfaces of the concaved bottom portions were very small, being of the order of a few thousandths of an inch, say about eight-thousandths inch. The blades were power driven and were fully rotary while the compartment was stationary. The attriting action of the device upon the aggregates was in the nature of a rubbing or smearing one, the aggregates bein the form of a fairly firm, cohesive dough. The latter was diluted with water to approximately 20% solids to give a very thick, smooth gel. The gel was allowed to dry down at room temperature until it had reached a moisture content of approximately The gel dried down to an ivory-like, distinctly opaque, very hard mass. This material machined very well, had the appearance of artificial ivory, and was unusually resistant to impact. If allowed to sit in water, however, it gradually softened and acquired first a rubbery and then a cheese-like consistency.

The following examples may illustrate the moisture resistance of several molded pieces in which a small amount of resin is incorporated.

Example 6 About 25 grams of a melamine-formaldehyde resin in powder form were stirred into 147 ml. of an aqueous solution of 0.5 normal HCl until dissolved, and the solution was then diluted to 208 ml. with distilled water. After standing for about 3 hours it was further diluted to 1250 ml. with distilled Water, forming a 2% solution of the resin. Then 250 cc. of the 2% solution were added to 2 liters of distilled water, with mixing, and after the resin had become well disposed, 95 grams of crystallite aggregates were added slowly and mixing continued for an hour. The aggregates were prepared by subjecting Ketchikan wood pulp to hydrolysis using a 0.5% by weight aqueous solution of HCl at 250 F. for one hour, there being produced cellulose crystallite aggregates having an average level-01f DR of 220, a moisture content, after drying, of about 1% by weight, and a particle size in the range of 1 to 250 to 300 microns. The resulting mixture was filtered through nainsook cloth with the aid of a slight vacuum, and the filter cake was broken up by hand, allowed to dry at room temperature, and then ground with a pestle and mortar. Thereafter it was pressed at a pressure of 10,000 psi for 15 minutes at a temperature of 230 F. The resulting disk, comprising 5% by weight of the resin and the balance crystallite aggregates, was able to resist the splitting action of water for 25 minutes, according to a test comprising placing the disc .in cold water (room temperature) and observing the time required for the first crack or fissure to appear in it. Such behavior was distinctly superior to that of a control disc containing no resin.

Example 7 Ten grams of vinyl resin fibers, comprising fibers made from a copolymer of 88% of vinyl chloride and 12% of vinyl acetate, were cut into inch lengths and then dissolved in 1 liter of acetone. Thereafter 90 grams of crystallite aggregates, as prepared in Example 6, were slowly added, with mixing, to the resin-acetone solution. After standing 1 hour, water was added to the mixture in quantities sufiicient to precipitate the resin on the aggregates. The mixture was then filtered through nainsook cloth, using a slight vacuum, and the filter cake was manually broken up and allowed to dry at room temperature. After preliminary grinding in a mortar and pestle, it was pressed under the conditions described in Example 6, the resulting pressed discs containing by weight of the resin. They resisted the splitting action of water for 150 minutes.

Example 8 Ketchikan sulfite wood pulp was subjected to hydrolysis using a 0.5% by weight aqueous solution of HCl at 250 F. for one hour, there being produced cellulose crystallite aggregates having an average level-off DR of 220 and, after drying, a moisture content of about 4% by weight. The aggregates had a particle size in the range of 1 to 250 to 300 microns. Then 100 parts by weight of a novolak, comprising a phenol-formaldehyde condensation product, was taken and, while in a thermoplastic condition, was mixed with 10 parts of a curing agent comprising hexamethylene-tetramine, after which the resin became thermosetting. The resulting mixture was mixed with the crystallite aggregates in a quantity sufiicient to produce a molding composition comprising by weight of aggregates and 10% by weight of the resin mix. This molding composition was used to form tan-colored molded discs, 2 inches in diameter by A; inch thick, in a conventional laboratory molding press using a temperature of 300 to 330 F., a pressure in the range of 2,000 to 4,000 p.s.i., and a molding time of 10 minutes. Upon testing the discs for 16 hours at 73 F., 50% RH, and a frequency of one megacycle, they exhibited a dielectric constant of 6.57; and after 72 hours testing at 73 F 90% RH, and one megacycle, the dielectric constant was 7.86. In a moisture resistance test, several discs were allowed to remain in boiling water for an hour and were found to be physically intact and to have picked up only 4% by weight of water. The discs had good impact strength.

Water-insoluble and/ or organic solvent-insoluble aggregates derivatives having the properties and characteristics described hereinbefore, such as, for example, carboxymethyl, hydroxypropyl or acetate derivatives, may be substituted for the cellulose crystallite aggregates in the foregoing examples to form products which are used in the same manner.

Of interest is the fact that the formed products, particularly those pressed at the lower pressures, exhibit oil adsorption properties. Useful articles such as bearing materials may be produced by forming a product into a desired shape and then letting .it soak in a suitable lubricant.

The flame resistance of the products suggests their use in electrical installations where are resistance is a requirement.

The products do not expand or contract to any appreci able extent with changes in temperature, and they are also very resistant to cold flow, or creep, under applied pressure.

A noteworthy property is the very high compressive strength of the products; thus, at the higher forming pressures, special forming molds are required to withstand the pressures. In view of the very great thermal and physical strength of the products, their use is contemplated for quick-opening molds capable of withstanding high temperatures and pressures, and which, if desired, may be made to be disposable.

Porous products may be made by forming a gel of the crystallite aggregates, introducing air thereto, and then pressing and drying. Such a material is of value in thermal insulation applications where light weight is a requi site.

It may be noted that the cellulose crystallite aggregates are a plastic material as they are capable of being formed by the application of pressure. More particularly, they may be said to have hydroplastic properties because, when pressed in the presence of water, the latter helps the aggregates to be formed or shaped more readily.

As indicated, several procedures are suitable for mechanically disintegrating the aggregates, a preferred one being to subject them to a high speed cutting action in the presence of an aqueous medium. Water is the preferred medium. Other suitable media are aqueous mixtures comprising water and one or more water miscible, oxygen-containing compounds such as the polyols, comprising aliphatic compounds having two or more hydroxy groups. These include di-, tri-, and polyhydric alcohols such as ethylene glycol, diethylene glycol, glycerol, triethylene glycol, propylene glycol, etc. Glycol ethers are suitable, such as Z-ethoXyethanol, Z-methoxyethanol, 2- butoxyethanol; also the monoethyl ether of diethylene glycol or the monobutyl or monomethyl ethers of this glycol. Other polyols are the polyoxyethylene glycols having an average molecular weight of 200 to 4000 or more, particularly 200 to 600; methoxy polyoxyethylene glycols having an average molecular weight of 350 to 550; esters of polyoxyethylene glycols of average molecular weight of 200 to 6,000; and diesters of the latter glycols having average molecular Weights of 1540 to 6,000. Other compounds are alkyl and alkylaryl ether derivatives of the polyoxyethylene glycols. Polyoxypropylene glycols of an average molecular weight of 150 to 425 are suitable, as are the water soluble polyoxyalkylene glycols, which contain recurring oxyethylene and oxypropylene group's. Water-miscible alcohols comprise a suitable class of compounds, and also organic acids, esters, aldehydes and ketones. Other compounds are ethers and oxides. Water may be omitted from any of the foregoing aqueous mixtures and the non-aqueous compound itself may be employed as the liquid medium in which the aggregates are disintegrated, particularly where 'such nonaqueous compound is both water soluble and liquid at room temperatures.

Although the invention has been described in connection with specific embodiments of the same, it will be understood that it is capable of obvious variations without departing from its scope.

In the light of the foregoing description, the following is claimed:

1. Method of producing a heat and impact-resistant hard plastic structure comprising subjecting cellulose crystallite aggregates having a particle size in the range of 1 to 300 microns to distintegrating action so as to reduce at least 1% by weight of the aggregates to a particle size less than one micron, and associating and shaping the distintegrated aggregates together in dense compact relationship with each other in the form of a unitary self-supporting three dimensional structure having a specific gravity in the range of 1.2 to 1.57.

2. The method of producing a heat and impact-resistant hard plastic structure as defined in claim 1 wherein the aggregates are subjected to a pressure sufficient to disintegrate at least 1% by weight of the aggregates to a particle size of less than 1 micron While confining the aggregates in the form of a unitary self-supporting three dimensional structure.

3. The method of producing a heat and impact-resistant hard plastic structure as defined in claim 1 wherein the aggregates, in the presence of water, are subjected to a pressure sufiicient to disintegrate at least 1% by weight of the aggregates to a particle size of less than 1 micron while confining the aggregates in the form of a unitary self-supporting three dimensional structure and then drying the structure.

4. The method of producing a heat and impact-resistant hard plastic structure as defined in claim 1 wherein the aggregates are subjected to distintegration in the presence of a liquid selected from the group consisting of water, water-miscible oxygen-containing organic compounds and mixtures thereof.

5. The method of producing a heat and impact-resistant hard plastic structure as defined in claim 4 wherein the liquid is Water.

6. The method of producing a heat and impact-resistant hard plastic structure as defined in claim 1 wherein the aggregates are subjected to disintegration in the presence of water, the disintegrated aggregates are dried and the dried disintegrated aggregates are pressed in the form of a unitary self-supporting three dimensional structure.

7. The method of producing a heat and impact-resistant hard plastic structure as defined in claim 1 wherein the aggregates are subjected to disintegration in the presence of water, the disintegrated aggregates-water mixture is subjected to pressure to remove excess water while confining the disintegrated aggregates in the form of a three dimensional structure and drying the structure to form a unitary self-supporting three dimensional structure.

References Cited by the Examiner UNITED STATES PATENTS 1,700,962 2/ 1929 Von Webern 264-299 1,815,959 7/1931 Wilderman 264-126 XR 2,779,974 2/1957 Billing et al 264123 XR 2,978,446 4/ 1961 Battista et a] 260-212 ALEXANDER H. BRODMERKEL, Primary Examiner.

P. E. ANDERSON, Assistant Examiner. 

1. METHOD OF PRODUCING A HEAT AND IMPACT-RESISTANT HARD PLASTIC STRUCTURE COMPRISING SUBJECTING CELLULOSE CRYSTALLITE AGGREGATES HAVING A PARTICLE SIZE IN THE RANGE OF 1 TO 300 MICRONS TO DISTINTEGRATING ACTION SO AS TO REDUCE AT LEAST 1% BY WEIGHT OF THE AGGREGATES TO A PARTICLE SIZE LESS THAN ONE MOCRON, AND ASSOCIATING AND SHAPING THE DISTINTEGRATED AGGREGATES TOGETHER IN DENSE COMPACT RELATIONSHIP WITH EACH OTHER IN THE FORM OF A UNITARY SELF-SUPPORTING THREE DIMENSIONAL STRUCTURE HAVING A SPECIFIC GRAVITY IN THE RANGE OF 1.2 TO 1.57. 